Area and edge effects in radiometric forces.

The radiometric force on several configurations of heated plates placed in a stagnant gas is examined experimentally, with a high-resolution thrust stand, and numerically using the direct simulation Monte Carlo method and a discrete ordinate solution of a model kinetic equation. A wide range of pressures from 0.006 to 6 Pa was examined, corresponding to Knudsen numbers from 20 to 0.02, in argon and helium test gases. The radiometric force, important in a number of emerging micro- and nanoscale applications, is shown to be mostly area dependent in the transitional regime where it reaches its maximum at Kn approximately 0.1.

Nonequilibrium molecular motion in a hypersonic shock wave.

Molecular velocities have been measured inside a hypersonic, normal shock wave, where the gas experiences rapid changes in its macroscopic properties. As first hypothesized by Mott-Smith, but never directly observed, the molecular velocity distribution exhibits a qualitatively bimodal character that is derived from the distribution functions on either side of the shock. Quantitatively correct forms of the molecular velocity distribution function in highly nonequilibrium flows can be calculated, by means of the Direct Simulation Monte Carlo technique.

Experimental verification of a technique for predicting scattered radiation transfer: application to low photon energies.

It is shown that predictions can be made of scatter-to-primary ratios for a variety of mammographic configurations. The different configurations can be produced by changes in source-detector distance; source-phantom distance; air gap; photon energy; phantom composition, thickness, and cross-sectional area. A detailed analysis of the effect of a grid is also considered. The predictions are not computationally intensive. Experimental verifications of the predictions for varying phantom cross-sectional dimensions, source-detector distance, and air gaps with and without a grid have been carried out. Variations with phantom thickness were not considered in this paper. Detailed comparisons between experiment and theory indicate that scatter-to-primary ratios were predicted within one standard deviation and the coefficient of variation is within 3% for most of the data points with the worst case coefficient of variation about 6%. Measurements of grid secondary transmission have also been compared to theoretical predictions and agreement is within the coefficient of variation of about +/- 15%.

The sensitivity to technological innovations of the doses delivered by optimized mammography systems.

Multiparameter optimizations have been carried out to study the effects of hypothetical technological advances on the patient doses required to maintain a given image quality in mammography. The assumed advances include: improvement in the power loading limits of the tube focal spot, increased absorption efficiency for a given detector resolution, increases in detector system gain, and changes in the exposure time limitations that result from patient motion. The optimization permits system geometry, kVp of the examination, filtration, detector resolution, focal spot size, and grid characteristics to vary simultaneously and self-consistently subject to image quality as well as technological constraints. The effects of technological innovations were measured by systematically varying the technological constraints to reflect hypothetical improvements and comparing the resulting minimized doses, required to maintain constant image quality, to a baseline optimized system derived from a set of baseline technological capabilities.

An approach to specifying a minimum dose system for mammography using multiparameter optimization techniques.

Analytical expressions have been written for image quality in mammography. Multiparameter optimizations have been conducted to find mammography systems providing the lowest patient dose for a given image quality. The optimizations are subject to constraints imposed by technology, such as power limits on the tube focal spot, absorption efficiency related to detector resolution, and others. The optimizations permit system geometry, kVp, filtration, detector resolution, focal spot size, and grid characteristics to vary simultaneously and self-consistently subject to the constraints. A system configuration approaching a factor of 3 dose reduction has been found without assuming radical technological advances. The system satisfies image quality constraints for both large and small targets and would be possible to implement clinically. The sensitivity of the results to the assumptions made in the modeling has been investigated.

On the significance of very small angle scattered radiation to radiographic imaging at low energies.

We have studied the angular distribution of scattered radiation at low energies for angles between 2 degrees and 15 degrees from the outward normal to the exit surface of several phantoms, with 1 degree resolution. A cryogenically cooled germanium detector was used to measure the spectra of the scattered radiation. The differential scattering fluences, or numbers of photons per unit solid angle per unit surface area, exhibit distinct peaks at angles in the vicinity of 5 degrees, with the angular position being only slightly energy dependent but very material dependent. The scattered spectra show large changes as a function of angle, in some cases actually becoming harder than the exit unscattered beam. The significance of this behavior relative to the imaging of targets in mammography examinations is discussed.

Experimental and theoretical energy and angular dependencies of scattered radiation in the mammography energy range.

Predicted scatter characteristics from a Monte Carlo code have been compared to an experiment for a 4.0-cm-thick acrylic resin (Plexiglas) phantom and 27.3-keV radiation. The results show generally good agreement, except that in the scattered spectra the predictions appear to underestimate the proportion of photons in the low-energy tail of the scattered radiation. The angular dependency of the energy-integrated scattered radiation fluence is in excellent agreement with the predictions.

A detailed experimental and theoretical comparison of the angular and energy dependencies of grid transmission.

Grid transmission characteristics have been predicted, with good agreement demonstrated between experiment and theory. Differences between the present work and an earlier theoretical analysis have been noted. It is shown that comparison between experiment and theory can lead to a sensitive method for determining grid parameters. Generally, the derived grid parameters deviate somewhat from manufacturer supplied data. Use of the manufacturer's nominal parameters can lead to large errors in predicting grid transmission.

An analysis of dose efficiency in image intensified fluoroscopic examinations.

In this paper we have developed expressions for assessing radiographic systems where grids, air gaps, or both are used. The application of these expressions to film-screen detectors gives predictions in good agreement with data that are found in the literature. Using this analytical tool, image intensified fluoroscopy has been studied. We have shown that under certain circumstances with air gaps there are potential gains in the efficient use of dose in fluoroscopy. The air gaps are of particular value where relatively thin body parts or small lesions are being examined using small focal spot generators. An independent gain control on the image intensifier would be useful for achieving the greatest flexibility when using magnification in fluoroscopy.

Mammography at reduced doses: present performance and future possibilities.

Reduced dose mammography is assessed with the aid of very recent work. It is concluded that there are technical and clinical reasons for believing that the reduced dose systems and their interpreters should perform more effectively than their predecessors. Review of known technologic achievements suggests a possible 6 to 7 reduction factor from present screen-film mammography dose levels without sacrificing image quality. Reasonable estimates of representative doses presently achieved by both xeroradiography and screen-film systems are given. The screen-film value is about 3 times higher than some previous estimates for these systems.

Focal spot size and scatter suppression in magnification mammography.

The improved image quality using magnification in film-screen mammography is related to a complicated interplay between focal spot size, x-ray tube output, detector resolution and sensitivity, air gap size, and target dimensions. Preliminary clinical evaluation of magnification mammography indicates that accuracy of diagnosis has been considerably enhanced. Additional technical and clinical evaluations are continuing, as suggested by mathematical modeling, in order to determine the best possible magnification protocols. While a small focal spot (less than 300 micrometers) is highly recommended for both contact and magnification with the film-screen systems, the smallest focal spot sizes (150 micrometers and smaller) may neither be necessary nor the best in all situations of reduced dose magnification mammography.

Relative carcinogenic effects of different mammography techniques.

Comparisons of different mammography techniques can be significantly altered by assumptions about the relevant measures for carcinogenic effects in mammography at low doses. Using a combination of recent data available from the literature, a study as been made of the significance of various assumptions concerning both the appropriate measures for carcinogenesis in mammography and the shape of the dose-effects relationship. It is found that for techniques typical of current reduced-dose mammography systems, the comparison of the relative carcinogenic effects of the techniques is significantly affected by both the measures that are used and the shape of th dose-effects relationship.

Analysis of the significance of scattered radiation in reduced dose mammography, including magnification effects, scatter suppression, and focal spot and detector blurring.

A systems analysis of reduced dose mammography with scatter removal has quantitatively specified the relationships between the several parameters in mammography systems. While the results are only preliminary in the sense that they need to be proven experimentally, they do seem to provide a good feel for the nature of the important trade-offs. The major uncertainties in the analysis probably have to do with the formal equivalence, that is implicit in the aperture theory, between large area and detail contrast, as well as the output versus focal spot relationship that was adopted. Also, there are important questions about specifying motion unsharpness as a function of exposure time in mammography examinations.

Optimization of electrostatic imaging systems for minimum patient dose or minimum exposure in mammography.

The x-ray photon energies that give the highest signal-to-noise ratios per unit of exposure or average dose in mammography are significantly greater than those used in current film/screen systems. Optimum photon energies for detection of calcifications in a range of thicknesses of water and fat were calculated. Soft-tissue targets were also considered. Comparison of the theoretical results and those obtained experimentally with the Stanton mammography phantom indicate that operation at less than optimum photon energies is a major cause of inefficiency in film/screen mammography. On the other hand, electrostatic imaging systems, because of their processing flexibility, can operate at the optimum photon energy levels, thus minimizing either dose or exposure.

Preliminary studies using electron radiography for mammography.

An electron radiography mammography system was used for phantom studies and to obtain 230 craniocaudal view patient studies, of which 140 were compared to xeroradiographs for their ability to visualize 7 features of diagnostic importance. The average exposure of the electron radiographs was 89 mR. The technique was considered equal or superior to xeroradiography in 58% of the compared diagnostic features, while patient dose was reduced.